TOKYO — Japan on Tuesday moved a mobile missile-defense system on the northern island of Hokkaido to a base near recent North Korean missile flyover routes.

Defense Minister Itsunori Onodera said a Patriot Advanced Capability-3 interceptor unit was deployed at the Hakodate base on southern Hokkaido "as a precaution" as part of government preparations for a possible emergency.

Japan currently has a two-step missile defense system. First, Standard Missile-3 interceptors on Aegis destroyers in the Sea of Japan would attempt to shoot down missiles mid-flight. If that fails, surface-to-air PAC-3s would try to intercept them.

Japan's Constitution, which limits the use of force to self-defense, only allows the military to shoot down missiles that are heading to Japan, or debris falling onto Japanese territory. Onodera has said a new security law passed in 2015 might allow it to shoot down a Guam-bound missile if it poses a critical security threat to Japan and its top ally, the United States.